1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly relates to a liquid crystal display device in which a reflective display and transmissive display are jointly used.
2. Description of the Related Art
Liquid crystal display devices have the advantages of thinness, light weight, and low power consumption compared with cathode ray tubes (CRT) and are utilized for personal computers, mobile phones, or other displays of electronic equipment.
Liquid crystal display devices are roughly divided into transmissive types and reflective types. Liquid crystal display devices differ from CRTs in that they are not self-luminescence type display devices. Therefore, a transmissive liquid crystal display device is provided with a planar light source called a “backlight” at its back surface and passes light from the backlight to a liquid crystal panel for display. Since a transmissive liquid crystal display device displays an image using the backlight, it remains unaffected even when the intensity of the ambient light is weak and can display an image with a high luminance and high contrast. However, the backlight consumes over 50% of the entire power consumption of a liquid crystal display device, so a transmissive liquid crystal display device suffers from the disadvantage that it is hard to reduce the power consumption. If the intensity of the ambient light is strong, a transmissive liquid crystal display device has the disadvantages that the display appears dark and the viewability deteriorates.
On the other hand, with a reflective liquid crystal display device, ambient light is used as the light source. The ambient light is received and reflected at the front by a reflecting plate etc. The reflected light is passed through the liquid crystal panel for display. The point light source constituted by the ambient light has to be converted to a planar light source on the display panel, so the reflecting plate has a rough surface so as to diffuse and reflect the light. Such a reflective liquid crystal display device differs from a transmissive liquid crystal display device in that it does not use a backlight, so it has the advantage of a lower power consumption. However, if the surroundings are dark, the reflected light is weak and so has a great effect. The disadvantages may arise of an insufficient luminance and contrast and deteriorated viewability. Particularly, in the case of color display, the efficiency of utilization of the reflected light ends up falling at the color filter, so the viewability remarkably deteriorates.
For overcoming the above disadvantages of a transmissive and a reflective liquid crystal display device, Japanese Unexamined Patent Publication (Kokai) No. 2001-318377 discloses a transflective liquid crystal display device using both transmission and reflection. A transflective liquid crystal display device displays an image by utilizing the reflection of ambient light in a bright place or utilizing a backlight in a dark place.
FIGS. 1A and 1B show a transflective liquid crystal display device of the related art. Specifically, FIG. 1A is a plane view of the surface of a first transparent substrate 101, and FIG. 1B is a cross-sectional view of the first transparent substrate 101 and a second transparent substrate 111 facing the first transparent substrate 101 along the line X-X of FIG. 1A.
As shown in FIGS. 1A and 1B, the transflective liquid crystal display device has the first transparent substrate 101, the second transparent substrate 111, a liquid crystal layer 131, a diffusion-reflecting region 102, a transparent region 103, and a color filter 140. The first transparent substrate 101 is facing and arranged to the second transparent substrate 111. The liquid crystal layer 131 is arranged between the first transparent substrate 101 and the second transparent substrate 111. The first transparent substrate 101 is formed with the transparent region 103 and the diffusion-reflecting region 102 in parallel.
The diffusion-reflecting region 102 of the first transparent substrate 101 is a region diffusing and reflecting ambient light. It diffuses and reflects front light incident from the second transparent substrate 111 side via the liquid crystal layer 131. When viewing a display formed by reflected light, ambient light incident with an angle of about 20 to 30 degrees with respect to the direction perpendicular to the surface of the first transparent substrate 101 is diffused and reflected at the front for enabling viewing of the display. The diffusion-reflecting region 102 diffuses and reflects light by forming on a flat first reflecting underlayer 121 formed on the first transparent substrate 101 a curved second reflecting underlayer 122 in an unevenness distributed configuration. The first reflecting underlayer 121 and the second reflecting underlayer 122 is covered by forming a flat reflecting film 123b and a curved bumpy reflecting film 123a. The surface is therefore formed with relief shapes. Note that, in the case of forming a thin film transistor LCD (hereinafter called a “TFT-LCD”), the reflecting film 123 of the diffusion-reflecting region 102 may be formed as reflective electrodes connected to drain electrodes of the TFTs using silver etc.
The transparent region 103 is a region passing light incident from the backlight. The backlight is provided at the other surface of the first transparent substrate 101 than the surface where the liquid crystal layer 131 is arranged. Light emitted from the backlight passes though the transparent region 103. Note that the transparent region 103 may be formed with transparent electrodes connecting to the drain electrodes of TFTs by using indium tin oxide (hereinafter called as “ITO”) in the case of a TFT LCD.
The color filter 140 is formed at the entire region of the second transparent substrate 111 facing the diffusion-reflecting region 102 and the transparent region 103 of the first transparent substrate 101. The color filter 140 passes and colors front light and back light.
The transflective liquid crystal display device of the related art is formed with the diffusion-reflecting region 102 and the transparent region 103 in parallel. The transflective liquid crystal display device of the related art has the diffusion-reflecting region 102 narrower than the reflective type and the transparent region 103 narrower than the transmissive type. Therefore, the transflective liquid crystal display device has the disadvantages of lower luminance and contrast and deteriorated viewability compared with a reflective or transmissive liquid crystal display device when used under ordinary conditions.
The diffusion-reflecting region 102 of the transflective liquid crystal display device of the related art reflects front light incident from the second transparent substrate 111 side via the liquid crystal layer 131 and displays the reflected light via the liquid crystal layer 131. Therefore, front light passes though the liquid crystal layer 131 a total of two times. On the other hand, the transparent region 103 passes the back light from the first transparent substrate 101 side to the liquid crystal layer 131 for the display. Therefore, the back light passes though the liquid crystal layer 131 just one time. In the case of color display, similarly, front light reflected at the diffusion-reflecting region 102 passes though the color filter 140 two times, while back light passing though the transparent region 103 passes though the color filter 140 one time. Therefore, when the liquid crystal layer 131 between the diffusion-reflecting region 102 and the transparent region 103 is a constant distance, the diffusion-reflecting region 102 has a light path longer than the transparent region 103. Therefore, a deterioration in luminance occurs. In particular, in color display, the color filter 140 may cause extreme deterioration in the viewability, insufficient color reproducibility, and unpractical display. Further, trying to obtain sufficient color reproducibility by providing the color filter 140 with in-plane differences in coloring concentration corresponding to the diffusion-reflecting region 102 and transparent region 103 requires a fine-machining and therefore a drop in the production efficiency.